Kaolin clay pigments used by the paper and paint industry are available in uncalcined (hydrated) and calcined (dehydrated) grades. When preparing aqueous coating or paper filling compositions containing calcined or uncalcined kaolin pigments, it is frequently desirable to provide the clay in the form of a concentrated aqueous suspension or slurry. The suspensions must be sufficiently fluid at both high and low rates of shear to be handled by conventional mixers and pumps.
When the clay is hydrated and has a limited content of particles larger that 2 microns (equivalent spherical diameter) it is relatively simple to produce stable, dispersed slurries containing up to about 70% clay solids on a weight basis. To manufacture these slurries a deflocculating agent (dispersant) such as tetrasodium pyrophosphate is added to clay in the form of a filter cake which typically contains about 60% solids. Additional dry clay is incorporated with mild agitation until a suspension having the desired high solids content is produced. The processing is conventionally referred to as "makedown." The resulting suspension of fine hyrated kaolin clay is stable in the sense that when it is allowed to stand, there is minimal settling of particles to form a dense sediment and there is minimal formation of a clear or cloudy supernatant liquid layer. This is attributable to the fact that suspensions of fine hydrated clay are fairly viscous and contain only small amounts of coarse particles. Few particles of clay, if any, have sufficient mass to settle under the influence of gravity.
In the case of clay pigments that contain significant amounts of course particles, especially particles larger than 2 microns, and the contents of ultrafine particles is low, there is a marked tendency of the course particles to settle out of deflocculated aqueous suspension of the clay. For example, 70% deflocculated suspensions of relatively coarse filler grades of hydrated clay tend to form hard sediments during shipment or storage. These filler clays usually contain at least 2% by weight of particles larger than 5 microns and at least 35% by weight of particles larger than 2 microns.
Calcined kaolin clay pigments generally have particle size distributions similar to those of high grade filler clays. High solids suspensions of calcined kaolin pigments therefore tend to form hard sediments during storage. Furthermore, deflocculated aqueous suspensions of calcined clay pigments have unusual rheological (viscosity) properties and they are very difficult to manufacture. Calcined kaolin clays usually cannot be prepared into suspensions containing more than about 60% clay by conventional techniques without producing systems which are excessively dilatant and resemble wet sands. When a ruler is dropped into a fluid concentrated slurry of calcined clay prepared by a conventional clay makedown procedure, it may be impossible to remove the ruler unless the ruler is removed very slowly. The shearing force applied to the suspension results in the conversion of the original fluid system to the dilatant mass which becomes increasingly viscous as the rate of shear increases. Processing equipment such as mixers and pumps would be damaged by such highly dilatant suspensions or the equipment would simply stop operating.